In Silico Evaluation of the Radical Scavenging Mechanism of Mactanamide.

Mactanamide (MA) is a diketopiperazine isolated from marine fungi of the genus Aspergillus. This compound is known as a natural antioxidant from experimental studies, yet this activity has not been successfully modeled thus far. In this work, the hydroperoxyl radical scavenging activity of MA was evaluated in the gas phase and physiological environments by thermodynamic and kinetic calculations. The results revealed that the HOO(•) radical scavenging of MA in the lipid media follows the formal hydrogen transfer mechanism via hydrogen abstraction at the O11-H bond. In the aqueous solution, however, the antioxidant activity follows the sequential proton loss electron transfer mechanism. The rate constant of the HOO(•) scavenging of MA in the polar environment is about 1045 times (k (overall) = 2.23 × 10(6) M(-1) s(-1)) higher than that in the lipid medium (k (overall) = 2.20 × 10(3) M(-1) s(-1)). In polar media, the HOO(•) radical scavenging activity of MA is therefore 18 times higher than that of Trolox, the reference compound when assessing antioxidant activity. The results presented here align well with the experimental data, validating the mechanistic pathways and thus providing useful insights into the antioxidant activity of MA.